How do the grades of compressed oxygen differ?

There are three grades of compressed oxygen gas widely sold in returnable supply cylinders: Aviation (aka ABO), Medical, and Industrial (aka Welding). The gas itself is normally produced using cryogenic separation of air, resulting in a gas that is very close to pure oxygen: > 99.5% with the balance being inert gases, mostly argon. Nearly all oxygen sold in the US is what is known as USP quality, meeting or exceeding United States Pharmacopeia quality standards for human consumption. As a practical matter, the differences in the three grades as sold today are not quality of the oxygen being put in the cylinder but rather the procedure used to fill the cylinder and, in the case of Medical grade, licensing or documentation requirements.

Industrial grade oxygen cylinders can be just topped up when refilling and, although unlikely, could be contaminated with other unknown gases that might have back-flowed into the cylinder during use or filling. Aviation and Medical grades must have the cylinder completely evacuated before every fill and each batch is tested for purity. Medical grade oxygen is legally regulated in the US by the Food and Drug Administration and filling a cylinder for medical use incurs some FDA licensing and documentation requirements for the cylinder fill station operator.

Medical oxygen is considered a drug, and thus requires a doctors prescription for the FDA licensed fill station operator to fill the cylinder. The FDA has granted an exception to the prescription requirement for medical cylinders marked as emergency use only, but they still should only be filled with USP quality oxygen and following FDA guidelines. To blend breathing gas mixtures for Nitrox and technical diving, most blenders use Aviation grade oxygen to avoid the FDA licensing and documentation requirements yet still use an oxygen grade suitable for human consumption.

Among divers, perhaps the most often debated topic about oxygen grades is should industrial grade oxygen be used for SCUBA applications. That's because welding oxygen is relatively cheap and brand name compressed gas suppliers usually fill oxygen cylinders from the same high quality source while following best practices regardless of grade. However, small welding supply operations are more likely to just top up their industrial grade oxygen cylinders from a local bulk supply and may not follow best practices. Unless the entire supply chain for each and every cylinder of oxygen can be trusted with absolute certainty, in our opinion using industrial grade oxygen for diving applications is a needless risk.

For much more detail about types of oxygen, especially recommendations regarding applications found in diving and diving emergencies, read Oxygen: Concentrations, Grades, and Labels by Larry "Harris" Taylor, Ph.D.